Since you're not using 2CO's button codes (which you shouldn't be, since you'd be using our Buy Now button codes for 2CO), I think the only possible "parameter error" could be with your 2CO Seller ID. Also double check that the 2CO Product ID in your product settings matches the ID for that product at 2CO's end.
There may be a problem with your 2CO account status which you should inquire with 2CO support about, but apparently you can substitute 2CO's test Seller ID "1303908" to at least see if your button will proceed to checkout. In this case the Product ID will reference a product in 2CO's test account, so don't worry if it doesn't match your product; you're only doing this to test if your button will proceed to checkout at all.
We really only support 2CO as a last-ditch fallback option for sellers who are unable to get a PayPal or Google Checkout account (Authorize.Net would be for US-based merchants who already have a card merchant account at their bank); if you're located in the US or UK and prefer not to use PayPal, we can recommend Google Checkout as your best option: